1. Field of The Invention
The present invention relates to a hydraulic shock absorber having a variable damping force characteristic structure and which is applicable to a vehicular suspension system. The present invention particularly relates to the hydraulic shock absorber in which the damping force characteristics at extension and compression (also called contraction) phases can be varied independently of each other.
2. Description of Background Art
A Japanese Utility Model (Registration) Application First Publication No. Showa 62-130241 exemplifies a first previously proposed hydraulic shock absorber in which both damping force characteristics at extension and compression phases are varied according to a rotational displacement position of a control valve element.
In the first previously proposed shock absorber, a control valve element (adjuster) is provided with an extension phase one-way check valve which is so constructed and arranged as to block a fluid flow from an extension/compression phase common bypass flow passage to an extension phase incoming flow passage direction and a compression phase one-way check valve is installed in a compression phase incoming flow passage so as to block the fluid flow from the extension/compression common bypass passage into a lower chamber. That is to say, the extension and compression phase one-way check valves are incorporated into an interior of a cylinder of the shock absorber so that an axial center bore of a piston rod is used as a common bypass passage at an extension phase (stroke) and compression phase.
Since, however, the axial center bore formed at an axial center portion of the piston rod is used as the extension/compression phase common bypass flow passage so that the flow passage and extension phase one-way check valve are disposed within the control valve element, an outer diameter of the control valve element cannot be smaller. Thus, the following problems occur:
(1) It is necessary to highly accurately finish a clearance between the outer diameter portion of the control valve element and an inner diameter portion of the piston rod in order to decrease deviations in the damping forces due to fluid leakage thereat. Thus, a cost of manufacturing the first previously proposed hydraulic shock absorber becomes accordingly high.
(2) Since the outer diameter of the control valve element is restricted, an outer diameter of a stud portion of the piston rod is accordingly restricted. Thus, depending upon a dimension in the piston rod, it becomes necessary to newly add the stud portion to the piston road as a separate part. Consequently, the cost of manufacturing the first previously proposed shock absorber becomes accordingly high.
(3) Since the outer diameter of the stud portion of the piston rod is restricted, a ratio of an outer diameter to an inner diameter of each of extension and compression phase high damping valves is accordingly restricted. Thus, a degree of freedom in tuning the damping force characteristics is restricted.
(4) Since the extension phase one-way check valve is needed to be disposed in the control valve element and the compression phase one-way check valve is needed to be disposed in the compression phase fluid incoming flow passage, the structure of the first previously proposed hydraulic shock absorber becomes complex and the cost thereof becomes accordingly high.
A U.S. Pat. No. 5,307,907 issued on May 3, 1994 exemplifies a second previously proposed hydraulic shock absorber.
In the second previously proposed hydraulic shock absorber, a hollow control rod has a central bore opening to a lower chamber and which communicates an upper chamber through axial slots, bypass bores, an angular passage, and radial passages. Since the central bore provides the extension/compression phase common flow passage D, the same problem of the restriction on the outer diameter of the control valve element occurs. Thus, the problems described above in the case of the first previously proposed hydraulic shock absorber are applied equally to those in the case of the second previously proposed hydraulic shock absorber.